The present invention generally relates to the formation of injection molded plastic structures, and more particularly relates to an improved cross-sectional configuration of a boss that is formed integrally with and projects outwardly from a side of a plate-shaped portion of an injection plastic molding.
One of the most difficult tasks encountered in the design of an injection molded plastic part is maintaining the integrity of cosmetic surface portions of the part while at the same integrally incorporating in the part structural portions that make the part functional. For example, on the flat, generally plate-shaped wall portions of many injection molded plastic parts there is an outer or cosmetic side surface that the user/consumer sees, and an inner or nominal side surface upon which protruding structural portions of the part, such as snaps, bosses, ribs and the like are formed. Each of these structural portions of the overall injection molded plastic part interrupts the flow of the plastic as it moves into the mold, and cause undesirable dimpling or "sinking" of the cosmetic side surface portion underlying the structural part if the shape of the structural protrusion is not carefully designed.
Webbed bosses are one of the most common structural protrusions formed on the nominal surface of a plate-shaped wall portion of an injection molded plastic part, and serve as locating features for parts to be mounted to the molded structure such as guide ribs, spacers, structural reinforcing members and the like. Preventing cosmetic side surface sink marks behind webbed bosses has always presented challenges to designers of injection molded plastic parts.
A general design rule of thumb in the formation of a protrusion on the nominal surface of a flat wall portion of an injection molded part is that to prevent cosmetic side surface sink behind the protrusion the maximum thickness of the protrusion at and parallel to the nominal surface should be no greater than about 60% of the wall thickness. In the case of a single rib formed on the nominal side surface, the cross-sectional thickness of the rib at the nominal surface can simply be made about 60% of the wall thickness to maximize rib strength without tending to create undesirable cosmetic side surface sink behind the rib.
However, when a multi-webbed boss of a conventional rectangularly cross-sectioned web configuration is to be formed on the nominal side surface of the molding wall portion, the cross-sectional thicknesses of the webs at the nominal side surface must be considerably less than 60% of the wall thickness to prevent cosmetic side surface sinking. This design limitation undesirably diminishes the strength of the webbed boss in both bending and torsional twisting modes. In view of this limitation it is seen to be desirable to provide an improved injection molded webbed boss design that increases the strength of the boss without increasing its tendency to cause cosmetic side surface sink beneath the boss. It is accordingly an object of the present invention to provide such an improved webbed injection molded plastic boss design.